Tetrafluoroethylene polymers are used to provide a protective coating around copper wire that is used for electrical wiring. The tetrafluoroethylene polymer is paste extruded around the wire by extruding a polymer paste through an extruder die around copper wire as the wire is passed lengthwise through an extruder.
The tetrafluoroethylene polymer is mixed with a lubricant, typically a petroleum distillate fraction such as naphtha, to form the polymer paste. The paste is extruded at temperatures typically between 90.degree. and 130.degree. F. Following extrusion the coated wire is raised in temperature first to dry off the lubricant, and next in progression to raise the polymer above its sinter temperature. The coated wire typically reaches 700.degree. to 750.degree. F. in the sintering stage.
In the extrusion process, air is present along the wire surface. At the high temperatures required for sintering the polymer, the oxygen in the air along the wire surface oxidizes the copper. Presence of this oxidized surface interferes with solderability of the insulated wire and with electrical conductivity of the wire.